The release notes for iOS 11.4 also outline a series of fixes and improvements:

Enables teachers to assign their students reading activities in iBooks using the Schoolwork app

Fixes an issue where certain character sequences could cause Messages to crash

Addresses a Messages issue that could cause some messages to appear out of order

Addresses an issue that could prevent logging in or accessing files on Google Drive, Google Docs and Gmail in Safari

Fixes an issue that could prevent data syncing in Health

Fixes an issue that could prevent users from changing what apps can access Health data

Resolves an issue that could cause an app to appear in an incorrect location on the Home screen

Fixes an issue where CarPlay audio could become distorted

Fixes an issue where selecting music from your iPhone could fail when playing music over Bluetooth or when connected to USB on some vehicles

To download the update, go Settings > General > Software Update (the preferred way, and it’s about 350MB), or connect your iPhone to a computer running iTunes and then carry out the update (this is slower and downloads the entire operating system as opposed to just the changes, and it comes in at several gigabytes).